1. Field of the Invention
The present invention relates to a trouble detection apparatus for an automobile air-bag system, and especially relates to a trouble detection apparatus for detecting an acceleration speed sensor and a trigger means for spreading out the air-bag.
2. Description of Prior Art
A conventional trouble detection apparatus for automobile air-bag system, for example shown in Japanese Published Examined Patent Application Sho 61-57219, is described referring to FIG. 3. FIG. 3 is a circuit diagram showing the conventional trouble detection apparatus of the air-bag. In FIG. 3, the air-bag system includes: an ignition switch 2 connected to a battery 1 of an automobile; an acceleration speed sensor 3 for detecting clashing of the automobile thereby closing switch 31; a resistor 32 connected across both contacts of the switch 31; a trigger means 4 for spreading out the air-bag by heating when an electric current is supplied thereto; another acceleration speed sensor 5 similar to the acceleration speed sensor 3; switch 51 and a resistor 52 respectively similar to the switch 31 and the resistor 32.
A trouble detection circuit 6 for detecting trouble coming in the acceleration speed sensors 3 and 5 and the trigger means 4 consists of a differential amplifier 7 and a comparator 8.
The differential amplifier 7 comprises four resistors 71, 72, 73 and 74 and an operational amplifier 75. The amplification degree of the differential amplifier 7 is determined by the values of the resistors 71 to 74. The comparator 8 comprises three resistors 81, 82 and 83, two operational amplifiers 84 and 85 and an AND gate 86. The resistors 81, 82 and 83 are used for generating reference voltages of the comparator 8.
Numeral 9 designates an alarm lamp for indicating occurrence of trouble by lighting due to an output signal issued from the comparator 8 when the trouble actually occurs.
The above-mentioned conventional trouble detection apparatus for automobile air-bag system has the following problem that: the amplification degree of the differential amplifier 7 is relatively large, around 100, so that it has a possibility of being influenced by noise; the differential amplifier 7 amplifies the D.C. voltage component besides necessary A.C. component, so that the undesirable error component of the D.C. voltage is also amplified; and it is necessary to pay attention to the input voltage of the operational amplifier 75 so as not to exceed its tolerance range.